1. Field of the Invention
This invention relates to the control of capacitance unbalance-to-ground in cables and, more particularly, to methods of and apparatus for the control of capacitance unbalance-to-ground of twisted conductor pairs in cables having conductors covered with at least a layer of cellular insulation by controlling the insulating of the conductors.
2. Prior Art
It is now common practice to fill the interstices among a plurality of twisted pairs of solid-plastic insulated conductors which comprise a core of a multipair telephone cable with a waterproofing compound to prevent the ingress of water which affects the electrical characteristics of the cable. Since the replacement of the air in the interstices with waterproofing compound results in poorer insulating properties, the amount of insulation about each conductor must be increased to retain the insulating properties of unfilled cable. This results in an increase in cross-sectional area of each of the insulated conductors, as well as that of the core, thereby requiring additional plastic compound to form a jacket about the core, all of which represents increased cost of manufacture. In order to realize the advantages of filled cable and, at the same time, to maintain the cost comparable to that of unfilled cable by reducing the size of the insulated conductors, individual conductors are insulated with a cellular plastic which may be covered with an outer skin of a solid abrasion-resistant plastic material. Because cellular plastic has a lower dielectric constant and hence, better insulating properties than solid plastic, a reduced wall thickness of cellular plastic insulation may be used with the result that the total thickness of the cellular plastic insulation plus the skin layer of solid plastic is equivalent to that for solid plastic insulation in an unfilled, air-core cable.
Although cellular plastic insulation is well known in the art, problems occur in the control of parameters, such as, for example, coaxial capacitance and outside diameter, of the insulated conductor, which are indicative of the quality of the insulated conductors. Coaxial capacitance is defined as the capacitance between the conductor and a probe which are separated by the insulation. Lack of control of the amount of expansion which is a parameter not generally encountered in solid plastic insulation can result in random variations in the dielectric constant of the cellular insulation. Random variations in the dielectric constant affects capacitance-to-ground, which is a cable characteristic is defined as the grounded capacitance of one insulated conductor of a twisted pair and is a measure of the quality of the conductor in its ability to resist the pick-up of energy from external sources, such as, for example, power transmission lines. See, for example, Horn, Y. W., "ABC of the Telephone Cable, Inside and Out", Vol. 5, 1974 Lee's ABC of the Telephone.
After the conductors are insulated with the cellular insulation, they are twisted together in pairs and subsequently formed into a cable. In order to be acceptable, twisted pairs of conductors must exhibit a difference or unbalance in capacitance-to-ground which is less than a predetermined value. Test results show that while the two conductors of any pair may each have acceptable coaxial capacitance and outside diameter values, when twisted together, the pair has unacceptable capacitance unbalance-to-ground values. It has been thought that if measured values of coaxial capacitance and outside diameter of each insulated conductor of a pair fell within acceptable limits, the capacitance unbalance-to-ground would also be acceptable. See Windeler, A. J., Polyethylene Insulated Telephone Cable, 4th Annual Wire and Cable Symposium, Dec. 7, 1955. This seeming anomaly may be overcome by twisting conductors having equal capacitance-to-ground values; however, this requires costly shop control.